Phytoplankton biomass and primary productivity were assessed on the continental shelf in the plume of the Amazon River during a series of cruises conducted within periods of minimum, maximum, rising and falling river discharge. Chlorophyll concentrations were greatest (up to 25.5 µg l­¹) in a zone located outside the turbid, high nutrient, low salinity riverine waters but shoreward of the clear, high salinity, low nutrient waters. Vertical distributions of chlorophyll further delineated the influences of these environmental regimes, with maximum chlorophyll concentrations occurring in the upper 5 m of water columns characterized by reduced salinities and elevated nutrients at the surface and low nutrient, high salinity water below. Fluorescence was elevated in the transition zone as a result of the phytoplankton standing stocks, and was also elevated in low-salinity waters influenced by the Amazon outflow. The residual fluorescence was coupled to salinity but not to chlorophyll, which suggested that it was related to dissolved organic matter which originated in the Amazon. Primary productivity on the continental shelf was greatest in the transition zone and occasionally exceeded 8 g C m-² d-¹. Productivity in the turbid, nutrient-rich waters and the clear, offshore regions averaged 2.18 and 0.81 g C m-² d-¹, respectively. Phytoplankton photosynthesis in waters influenced by the Amazon River appeared to be limited by low levels of available irradiance inshore, whereas offshore it was ¹nutrient-limited. The narrow zone of high production was supported by the riverine input of nutrients and the dynamics of sediment flocculation. Removal of the inorganic sediment load was necessary to allow for adequate irradiance penetration to support photosynthesis.